Ultrasound imaging apparatuses are widely used, for medical diagnosis purposes, to obtain an image showing the internal organs of a human body. The ultrasound imaging apparatus transmits ultrasound signals toward a target object to be diagnosed, and utilizes the echo signals reflected from the target object to obtain an image showing the desired target object. One of the obstacles encountered with a conventional ultrasound imaging apparatus in visualizing pathological tissues, such as tumors and cancerous tissues, is that if reflectivity between the pathological tissues and the neighboring tissues are not significantly different, then the conventional ultrasound imaging apparatus cannot provide a clear enough image of the pathological tissues for diagnosis. Conventionally, to overcome the above obstacle, a method of visualizing the elastic characteristics of an entire medium, including the pathological tissue and its neighboring area, is used to discriminate between the pathological tissue and the normal tissue. The method exploits the pathological tissue's property of having a greater stiffness than the neighboring normal tissues.
Two kinds of methods for measuring the elastic characteristics of a medium are known in the art: one of the methods is to externally apply pressure to a medium of interest, measure the physical displacement of the medium, and calculate the local displacement from the pressure applied to the medium; and another exploits the difference in wave transmission properties between a stiff medium and a soft medium, and consists of the steps of applying low frequency vibrational forces to a medium and visualizing movement of the applied low frequency wave. However, all these methods have drawbacks as explained below.
In the first method, a relative displacement must be estimated between the echo signals received in response to transmission of an ultrasound signal to the medium after applying pressure thereto and before applying pressure to the medium, in order to diagnose if the medium is pathological tissue. Preferably, the pressure applied to the medium is increased since accurate diagnosis requires information on displacements in response to a wide range of pressures. However, if the pressure is increased, the correlation between the echo signals before and after applying pressure decreases, such that correctly identifying the displacement is difficult. As a result, discriminating between normal and pathological tissues is not viable. Further, in both methods, calculating waveform displacements or phase changes based on RF or envelope data, relating to the transmitted and received ultrasound signals is necessary. Since these methods need to compute signal correlations, they require complex hardware having high-speed signal processing capability, and thus the costs associated with these methods is high.
Accordingly, a need for a new and improved method and apparatus for measuring the elastic characteristics of a medium exists in the art.